Method and system for determining a state of charge of a battery

ABSTRACT

Methods and systems are provided for determining a state of charge of a battery. The battery is subjected to a predetermined magnetic field such that the battery and the predetermined magnetic field jointly create a resultant magnetic field. The resultant magnetic field is sensed. The state of charge of the battery is determined based on the resultant magnetic field.

TECHNICAL FIELD

The present invention generally relates to electrochemical powersources, such as batteries, and more particularly relates to a methodand system for determining the state of charge of a battery.

BACKGROUND OF THE INVENTION

In recent years, advances in technology, as well as ever-evolving tastesin style, have led to substantial changes in the design of automobiles.One of the changes involves the complexity, as well as the power usage,of the various electrical systems within automobiles, particularlyalternative fuel vehicles, such as hybrid, electric, and fuel cellvehicles.

Such vehicles often use electrochemical power sources, such asbatteries, ultracapacitors, and fuel cells, to power the electric motorsthat drive the wheels, sometimes in addition to another power source,such as an internal combustion engine. An important parameter in theoperation of vehicles that utilize batteries is the “state of charge”(SOC). The state of charge refers to the stored energy in the batterythat is available to be used at any given time relative to the storedenergy that is available when the battery is fully charged. An accuratedetermination of the state of charge allows for the vehicles to maximizeperformance and efficiency while minimizing emissions.

A conventional approach for batteries is to relate either a measured orcalculated open circuit voltage to the state of charge. This is feasiblebecause the open circuit voltage, which is the resting voltage of thebattery when no load is applied, generally exhibits some observabledependence on the battery's state of charge. There are batteries,however, such as nickel metal hydride and some types of lithium ionbatteries, which possess a nearly constant open circuit voltage acrossmost of the range of state of charge. In other words, the open circuitvoltage reveals nothing about the state of charge of the battery.Therefore, while these batteries are highly desirable as power sourcesfor electric and hybrid vehicles because of their low mass, high powercapability, and large energy storage capacity, they present a problemwith regard to control because it is very difficult to estimate theirstate of charge with any degree of certainty.

Accordingly, it is desirable to provide a method and a system fordetermining the state of charge of a battery that is not based on itsopen circuit voltage. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description and the appended claims, taken inconjunction with the accompanying drawings and the foregoing technicalfield and background.

SUMMARY OF THE INVENTION

A method for determining a state of charge of a battery is provided. Thebattery is subjected to a predetermined magnetic field such that thebattery and the predetermined magnetic field jointly create a resultantmagnetic field. The resultant magnetic field is sensed. The state ofcharge of the battery is determined based on the resultant magneticfield.

A method for determining a state of charge of an automotive battery isprovided. A predetermined magnetic field is generated. The battery issubjected to the predetermined magnetic field such that the battery andthe predetermined magnetic field jointly create a resultant magneticfield. A magnetically responsive component is subjected to the resultantmagnetic field. An electromagnetic response within the magneticallyresponsive component is detected. The state of charge of the battery isdetermined based on the electromagnetic response.

An automotive drive system is provided. The automotive drive systemincludes an electric motor, a battery coupled to the electric motor, amagnetic field generator, a magnetically responsive component, and aprocessor in operable communication with the magnetically responsivecomponent. The magnetic field generator is configured to subject thebattery to a predetermined magnetic field such that the battery and themagnetic field generator jointly create a resultant magnetic field. Themagnetically responsive component is arranged and configured to generatean electromagnetic response to the resultant magnetic field. Theprocessor is configured to detect the electromagnetic response anddetermine the state of charge of the battery based on theelectromagnetic response.

DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a schematic view of an exemplary automobile according to oneembodiment of the present invention;

FIG. 2 is an isometric view of a state of charge (SOC) determinationsystem within the automobile of FIG. 1 according to one embodiment ofthe present invention;

FIG. 3 is a schematic circuit diagram of the SOC determination system ofFIG. 2;

FIG. 4 is an isometric view of a SOC determination system according toanother embodiment of the present invention;

FIG. 5 is a cross-sectional side view of the SOC determination system ofFIG. 4 taken along line 5-5;

FIG. 6 is a schematic view of a Hall Effect sensor within the SOCdetermination system of FIG. 4;

FIG. 7 is a flow chart of a method for determining a state of charge ofa battery according to one embodiment of the present invention; and

FIG. 8 is a graph comparing a weight factor to a current flow through abattery used in the method of FIG. 7.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

The following description refers to elements or features being“connected” or “coupled” together. As used herein, “connected” may referto one element/feature being directly joined to (or directlycommunicating with) another element/feature, and not necessarilymechanically. Likewise, “coupled” may refer to one element/feature beingdirectly or indirectly joined to (or directly or indirectlycommunicating with) another element/feature, and not necessarilymechanically. However, it should be understood that although twoelements may be described below, in one embodiment, as being“connected,” in alternative embodiments similar elements may be“coupled,” and vice versa. Thus, although the schematic diagrams shownherein depict example arrangements of elements, additional interveningelements, devices, features, or components may be present in an actualembodiment. It should also be understood that FIGS. 1-8 are merelyillustrative and may not be drawn to scale.

FIG. 1 to FIG. 8 illustrate a method and system for determining a stateof charge of a battery. The battery is subjected to a predeterminedmagnetic field such that the battery and the predetermined magneticfield jointly create a resultant magnetic field. The resultant magneticfield is sensed. The state of charge of the battery is determined basedon the resultant magnetic field.

The sensing of the resultant magnetic field may include subjecting amagnetically responsive component (e.g., a conductive wire or a piecemetal or semiconductor material) to the resultant magnetic field anddetecting an electromagnetic response (e.g., current, voltage, or power)within the magnetically responsive component.

FIG. 1 illustrates a vehicle, or automobile, 10 according to oneembodiment of the present invention. The automobile 10 includes achassis 12, a body 14, four wheels 16, and an electronic control system(or electronic control unit (ECU)) 18. The body 14 is arranged on thechassis 12 and substantially encloses the other components of theautomobile 10. The body 14 and the chassis 12 may jointly form a frame.The wheels 16 are each rotationally coupled to the chassis 12 near arespective corner of the body 14.

The automobile 10 may be any one of a number of different types ofautomobiles, such as, for example, a sedan, a wagon, a truck, or a sportutility vehicle (SUV), and may be two-wheel drive (2WD) (i.e.,rear-wheel drive or front-wheel drive), four-wheel drive (4WD) orall-wheel drive (AWD). The automobile 10 may also incorporate any oneof, or combination of, a number of different types of engines (oractuators), such as, for example, a gasoline or diesel fueled combustionengine, a “flex fuel vehicle” (FFV) engine (i.e., using a mixture ofgasoline and alcohol), a gaseous compound (e.g., hydrogen and/or naturalgas) fueled engine, or a fuel cell, a combustion/electric motor hybridengine, and an electric motor.

In the exemplary embodiment illustrated in FIG. 1, the automobile 10 isa hybrid vehicle (or hybrid electric vehicle (HEV)), and furtherincludes an actuator assembly (or powertrain) 20, a battery 22, abattery state of charge (SOC) determination system 24 (or “SOC system24”), a power inverter (or inverter) 26, and a radiator 28. The actuatorassembly 20 includes an internal combustion engine 30 and an electricmotor/generator (or motor) system (or assembly) 32. The battery 22 iselectrically connected to the inverter 26 and, in one embodiment, is alithium ion (Li-ion) battery including a plurality of cells, as iscommonly understood.

Although not illustrated, the power inverter 26 may include a pluralityof switches, or transistors, as is commonly understood. The electricmotor system 32, in one embodiment, includes one or moresinusoidally-wound, three-phase alternating current (AC)motor/generators (or motors) (e.g., permanent magnet) such as commonlyused in automotive vehicles (e.g., traction drive control systems andthe like). As will be appreciated by one skilled in the art, each of theelectric motors includes a stator assembly (including conductive coils),a rotor assembly (including a ferromagnetic core), and a cooling fluid(i.e., coolant). The stator assembly and/or the rotor assembly withinthe electric motors may include multiple (e.g., sixteen) electromagneticpoles, as is commonly understood.

Still referring to FIG. 1, and as described in greater detail below, thecombustion engine 30 and the electric motor system 32 are integratedsuch that one or both are mechanically coupled to at least some of thewheels 16 through one or more drive shafts 34. In one embodiment, theautomobile 10 is a “series HEV,” in which the combustion engine 30 isnot directly coupled to the transmission, but coupled to a generator(not shown), which is used to power the electric motor 32. In anotherembodiment, the automobile 10 is a “parallel HEV,” in which thecombustion engine 30 is directly coupled to the transmission by, forexample, having the rotor of the electric motor 32 rotationally coupledto the drive shaft of the combustion engine 30.

The radiator 28 is connected to the frame at an outer portion thereofand although not illustrated in detail, includes multiple coolingchannels throughout that contain a cooling fluid (i.e., coolant) such aswater and/or ethylene glycol (i.e., “antifreeze) and is coupled to theengine 30 and the inverter 26. The inverter 26 receives and sharescoolant with the electric motor 32. The radiator 28 may be similarlyconnected to the inverter 26 and/or the electric motor 32.

The SOC system 24 includes a magnetic sensor 36 and a SOC module 38. Themagnetic sensor 36 is located adjacent (or connected to) the battery 22(or more particularly one cell of the battery 22). The SOC module 38 isin operable communication with the magnetic sensor 36 and in oneembodiment includes at least one processor and/or a memory that includesdata relating a magnetic property of the battery 22 to the state ofcharge of the battery 22, as is described in greater detail below.Although not illustrated as such, the SOC module 38 may be integral withthe electronic control system 18 and may also include one or more powersources.

FIGS. 2 and 3 illustrate the SOC system 24, according to one embodimentof the present invention, in greater detail. In the embodiment shown inFIG. 2, the magnetic sensor 36 includes a first (or primary) conductivewinding (or coil) 40 and a second (or secondary) conductive winding 42.The first and second conductive windings 40 and 42 are made ofelectrically conductive wire, such as copper wire, and are wrappedaround respective first and second portions of the battery 22. The firstand second portions of the battery 22 may overlap, and as such, thefirst and second conductive windings 40 and 42 overlap or areintertwined. As shown schematically in FIG. 3, within the SOC module 38(not shown), the SOC system 24 also includes a current source 44, avariable inductor 46, a variable capacitor 48, variable resistors 50, adifferential amplifier 52, and a phase sensitive detector amplifier 54.

Referring again to FIG. 1, the electronic control system 18 is inoperable communication with the actuator assembly 20, the battery 22,the SOC system 24, and the inverter 26. Although not shown in detail,the electronic control system 18 includes various sensors and automotivecontrol modules, or electronic control units (ECUs), such as a bodycontrol module (BCM) 19, and at least one processor and/or a memorywhich includes instructions stored thereon (or in anothercomputer-readable medium) for carrying out the processes and methods asdescribed below.

During operation, still referring to FIG. 1, the automobile 10 isoperated by providing power to the wheels 16 with the combustion engine30 and the electric motor assembly 32 in an alternating manner and/orwith the combustion engine 30 and the electric motor assembly 32simultaneously. In order to power the electric motor assembly 30, DCpower is provided from the battery 22 to the inverter 26, which convertsthe DC power to AC power, prior to energizing the electric motor 32.

As will be appreciated by one skilled in the art, at various stages ofoperation, it is beneficial to have an accurate estimate of the state ofcharge of the battery 22, particularly in an embodiment using a lithiumion battery. According to one embodiment of the present invention, themagnetic sensor 36 detects, or senses, a magnetic property of thebattery 22. The SOC module 38 (and/or the electronic control system 18)then determines the state of charge of the battery 22 based, at least inpart, on the magnetic property. In one embodiment, the magnetic propertyis magnetic susceptibility which is determined by detecting anelectromagnetic response within the second conductive winding 42. Theelectromagnetic response is caused by a magnetic field generated byflowing current through the first conductive winding 40 in combinationwith a magnetic field generated by the battery 22 in response to themagnetic field generated by the first conductive winding 40.

Magnetic susceptibility describes the extent to which a material becomesmagnetized in the presence of an applied magnetic field. The magneticsusceptibility per unit volume of the material, χ_(v), is given by theequation

$\begin{matrix}{{\chi_{v} = \frac{M}{H}},} & (1)\end{matrix}$where M is the magnetization expressed as the magnetic dipole per unitvolume, and H is the applied magnetic field. Susceptibility may also beexpressed per unit mass or per mole of the material. The mechanicalforce exerted by the applied magnetic field on the material isproportional to the susceptibility χ, to the magnetic field strength,and to the magnetic field gradient. If χ is positive, the material isattracted to regions of increasing magnetic field strength and isdescribed as being “paramagnetic.” If χ is negative, the material isconversely repelled and is described as being “diamagnetic.”

The magnetization induced in the material by the action of the appliedmagnetic field generates its own magnetic field that combines with theapplied field. In the case of a paramagnetic material, the combinedmagnetic field is in general increased over the applied magnetic fieldalone, where that increase is proportional to the paramagneticsusceptibility of the material. In the case of a diamagnetic material,the resulting combined magnetic field is, conversely, reduced. Bothcases can, in principle, be used for the purposes of this invention, butbecause paramagnetism is a much stronger phenomenon than diamagnetism,in general, the former is preferred.

Apart from magnetism generated by free circulating electrical currents,as in, e.g., electromagnets, magnetism in materials generally arisesfrom both the localized spin of electrons and their orbital motionwithin atoms. Magnetic susceptibility is observed in free ions of theiron-group series, actinide-series, and rare-earth series elements onthe periodic table. Compounds incorporating these elements also exhibitsusceptibility, and some of these compounds find use as active materialsfor electrochemical energy storage in batteries. They often belong to aclass known as intercalation compounds, which are characterized by theability to have small ions (such as Li) readily inserted into andwithdrawn from their solid-state structures. This behavior provides forthe charge and discharge processes of the batteries.

Common metal oxides for lithium ion batteries that are intercalationmaterials include lithium cobalt oxide (LiCoO₂), lithium nickel oxide(LiNiO₂), and variants of the form LiCo_(x)Ni_(y)Mn_(z)O₂, where thecobalt, nickel, and manganese species occupy the same lattice andx+y+z=1. On the other hand, some materials form two phases and arereferred to as simply insertion electrodes, a more general term thatalso comprises intercalation materials. An example of a two-phaseinsertion electrode presently being considered for use as a cathode inlithium ion batteries is iron phosphate (FePO₄). The relevantelectrochemical reaction isLi_((1-n))FePO₄ +nLi⁺ +ne ⁻=LiFePO₄,  (2)where n is the number of lithium ions and electrons involved in thereaction. During discharge of the battery, lithium is inserted into theiron phosphate, and while the battery is being charged, lithium isremoved. The fraction of lithium in the material relative to the maximumamount of lithium the material can except (i.e., one Li in LiFePO₄)corresponds to the fractional state of charge, which when multiplied by100 yields the state of charge.

When the free atoms, iron (Fe), phosphorous (P), and oxygen (O), in ironphosphate join, the individual electronic structures are modified tobecome part of the larger compound. The valence electrons of each atomcontribute to bonding within the compound and charge transfer occursamong the atoms. The new electronic structure that is formed ischaracteristic of the specific compound and has a unique magneticsusceptibility associated with it. Further modification of theelectronic structure occurs when more ions are introduced to thecompound, as would be the case with insertion of lithium into the ironphosphate electrode during discharge of a lithium ion battery. Thischange has a measurable effect on the susceptibility of the electrode inproportion to the amount lithium added. By systematically varying thefraction of lithium in the electrode and measuring the correspondingsusceptibility, χ, it is possible to establish a relationship betweenthe two variables. Embodiments of the present invention utilize changesin the magnetic susceptibility of the electrode to determine the stateof charge of the battery.

The SOC module 38 conducts current through the first conductive winding40 to generate a predetermined magnetic field. As will be appreciated byone skilled in the art, changes in the magnetic field cause anelectromagnetic response within the second conductive winding 42, suchas a voltage, current flow, and/or electric power. At the same time, thepredetermined magnetic field causes the battery 22 to become magnetized,and thus generate a complimentary magnetic field, based on its state ofcharge, which interacts with the predetermined state of charge andsimilarly causes an electromagnetic response (e.g., current, voltage,and/or power) in the second conductive winding 42. Stated differently,the predetermined magnetic field and the complimentary magnetic fieldinteract to jointly form a resultant (or total) magnetic field whichcauses an electromagnetic response within the second conductive winding42 based on the state of charge of the battery 22.

Thus, prior to operation, the battery 22 may be connected to a “cycler”(or charger) and the state of charge of the battery 22 may be set todesired values while the electromagnetic response within the secondconductive winding 42 is monitored. The information gathered is used togenerate a look-up table, a graph, and/or a function of the strength ofthe electromagnetic response within the second conductive winding 42 vs.state of charge of the battery 22, which is stored in the SOC module 38.The data in the look-up table may be taken from laboratory experimentsperformed on representative batteries across the range of expectedoperating temperatures. Algorithms for controlling the hardware,deciding when to make measurements, and processing the signal from thehardware may also be stored in the SOC module 38.

During normal operation, referring to FIGS. 2 and 3, as the state ofcharge of the battery 22 changes, the electromagnetic response withinthe second conductive winding 42 varies. The electromagnetic response isdetected by the SOC module 38, which compares it to the states ofcharges within the look-up table to determine the present state ofcharge of the battery. In at least one embodiment, the state of chargeis determined without using the voltage of the battery 22.

Referring specifically to FIG. 3, the variable inductor 46 may beadjusted to give a signal that balances that of the second conductivewinding 42, and the variable resistors 50 allow for fine tuning of thesignal into the differential amplifier pre-amplifier 52 of the phasesensitive detector (lock-in) amplifier 54. The variable capacitor 48across the variable inductor 48 serves to adjust the phase shift of thesignal within the first conductive winding 40 relative to that withinthe second conductive winding 42 for optimal balancing of the two signalinputs to the amplifier 52.

FIGS. 4, 5, and 6 illustrate the SOC system 24 according to anotherembodiment of the present invention in which the magnetic sensor 36includes a permanent magnet 56 and a Hall Effect probe (or sensor) 58.As shown in FIG. 5, the permanent magnet 56 is positioned adjacent to aside 60 of the battery 60 such that a dashed line 62 extending throughthe poles (N and S) thereof is parallel to the side 60. As a result,magnetic flux lines 62 extending from the magnet 56 extend into thebattery 22 in a direction that is substantially perpendicular to theside 60 of the battery 22.

Still referring to FIGS. 4 and 5, the Hall Effect probe 58 includes aprobe end 66 that is positioned on the side 60 of the battery 22adjacent to the magnet 56. As shown schematically in FIG. 6, the HallEffect probe 58 includes a magnetically responsive component 68, such asa conductor or semiconductor, through which current (I) is conducted bya current source (not shown) and oriented relative to the magnet 56 suchthat the flux lines 62 extend therethrough as shown. As will beappreciated by one skilled in the art, when a magnetic flux (e.g., fluxlines 62) passes through the component 68 substantially perpendicularly,the electrons flowing through the component 68 are forced to one side ofthe component 68, thus creating an electromagnetic response voltagewithin the component 68 (i.e., a Hall Voltage (V_(H)) across the sidesof the component transverse to the flow of current).

A predetermined magnetic field generated by the permanent magnet 56causes the battery 22 to become magnetized, and thus generate acomplimentary magnetic field, based on its state of charge, whichinteracts with the predetermined state of charge and similarly causes anelectromagnetic response in the magnetically responsive component 68within the Hall Effect probe 58. Stated differently, the predeterminedmagnetic field and the complimentary magnetic field interact to jointlyform a resultant (or total) magnetic field which causes anelectromagnetic response within the component 68 based on the state ofcharge of the battery 22.

As the magnetic field of the battery 22 changes with its state ofcharge, the voltage detected across the component 68 changes as theamount of magnetic flux passing perpendicularly therethrough is altered.In a manner similar to that described above, a look-up table may begenerated that compares detected Hall voltages to known states of chargeof the battery 22. The look-up table is stored within the SOC module 38and used to determine the state of charge of the battery 22 using thevoltage detected within the Hall Effect probe 58 during normal operationof the automobile 10.

The state of charge may be measured continuously throughout operation,or if electromagnetic interference makes this impractical, it may bemeasured at times when there is little or no current flowing in thebattery 22. For hybrid (or electric) vehicle batteries, it may bepreferable to take measurements while the automobile 10 is off, or justafter the ignition is activated.

In one embodiment, whenever current is within acceptable limits and avalid susceptibility measurement becomes available, the correspondingstate of charge (or a magnetic portion of the state of charge), SOC_(χ),is retrieved from the look-up table and is blended as a correctionsignal with a current associated with the battery, such as anamp-hour-based state of charge (or a current portion of the state ofcharge), SOC_(Ah), which may correspond in an integration of the currentflow through the battery 22. The blending is performed in a manner suchthat the weighting of the susceptibility-based state of charge (SOC_(χ))is greatest at the point at which the measurement occurs, thendiminishes as a function of the amp-hour throughput (ΔAh) that thebattery experiences. This can achieved through the use of a variableweight factor, w, in the blending equationSOC=wSOC _(χ)+(1−w)SOC _(Ah),  (3)where w is a function of ΔAh. The amp-hour-based state of charge iscalculated continuously whenever current is flowing through the battery22 in either the SOC module 38, or another, remote microprocessor whichtransmits the value over a vehicle communication bus. While a linearcombination is proposed in Eq. 3, other combinations (e.g., a geometricmean, or SOC=SOC=sqrt(SOC_(χ)SOC_(Ah)) may also be employed).

The amp-hour-based state of charge is calculated according to theequation

$\begin{matrix}{{{SOC}_{A\; h} = {{SOC}_{t - {\Delta\; t}} + {\frac{I\;\Delta\; t}{A\; h_{nominal}} \times 100}}},} & (4)\end{matrix}$where I is current, Δt is the time interval of the calculation, andAh_(nominal) is the rated capacity of the battery. Note that theincremental change in

${SOC}_{A\; h},{\frac{I\;\Delta\; t}{A\; h_{nominal}} \times 100},$is added to the blended SOC from the previous time step, SOC_(t-Δt). Inthis manner, the calculation of SOC_(Ah) may always incorporate anycorrection that has been provided by SOC_(χ). By allowing the influenceof SOC_(χ) to decay by means of the weight factor w, the reported stateof charge is not unduly biased by data that is too old to be applicable.Other methods of calculating state of charge based on current (and/orvoltage) are known in the art, such as those described in U.S. Pat. No.6,639,385, and may be utilized in other embodiments of the presentinvention in combination with the methods described herein.

FIG. 7 illustrates a method 70 for determining a state of charge,according to one embodiment of the present invention. The series ofsteps 72-86 may occur continuously at intervals of one second or lesswhenever the automobile 10 is turned on.

The method 70 begins at step 72 when the vehicle key is turned on (e.g.,when the ignition is activated), and values of state of charge(SOC_(old)) and amp-hour throughput (ΔAh_(old)) that were stored whenthe vehicle last powered down are retrieved from non-volatile memory. Atstep 74, current is measured and the amount of energy that has passedthrough the battery since the magnetic susceptibility (χ) was lastdetermined (ΔAh) is updated. At step 76, a test is performed todetermine whether the current flow through the battery pack is lowenough (i.e., within a preset limit stored in the SOC module 38) suchthat it will not interfere with the magnetic sensor 36. If thiscondition is met, the electromagnetic response of the magneticallyresponsive component (e.g., the second conductive winding 42 or thecomponent 68) is measured at step 78 and converted to asusceptibility-based state of charge (SOC_(χ)) by means of the look-uptable stored in the SOC module 38 (FIG. 2) at step 80. Then, at step 82,ΔAh is reset to zero.

At step 84, the weight factor (w) for the contribution of SOC_(χ) to theblended state of charge is taken from a look-up table which describes arelationship between w and ΔAh. An example of such a table is shown inFIG. 8. Note that if ΔAh has just been reset to zero, w will take on itsmaximum value as indicated by the exponential decay of was shown by line88. At step 86, the blended state of charge is calculated using theweight factor determined in step 84 (e.g., 0.5). The method 70 thenloops back to step 74 and is repeated.

At step 76, if the current is not within the preset limits, the method70 bypasses steps 78, 80, and 82 and proceeds with step 84. For eachsuccessive loop through the process for which the current conditionfails to be met, the value of ΔAh will increment due to the operation ofthe automobile 10, and the corresponding value of w, retrieved from thelook-up table decreases according to FIG. 8. In this manner, theinfluence of SOC_(χ) on state of charge is prominent whenever thecurrent condition is met, and fades during periods when the currentcondition is not met.

One advantage of the method and system described above is that becausethe state of charge of the battery is determined without using thevoltage of the battery, the use of batteries with relatively invariantopen circuit voltage, such as some lithium ion batteries, isfacilitated. Another advantage is that because of the weighting schemedescribed above, the magnetic state of charge is always supplemented bythe current-based state of charge. Thus, the accuracy of the state ofcharge calculation is improved.

Other embodiments may be directed towards sensing or detecting magneticproperties of the battery other than magnetic susceptibility, such asmagnetization, magnetic moments, and magnetic permeability with any typeof sensor capable of sensing the property. The method and system may beused in vehicles other than automobiles, including aircraft andwatercraft, as well as other types of electrical systems utilizingelectrochemical power sources, such as computing systems. Anelectromagnet or any device capable of generating field that stimulatesa magnetic response in the battery may be used instead of the permanentmagnet shown. Other methods for combining the magnetic portion and thecurrent portion of the state of charge may be used. It is also possiblefor voltage-based methods to be combined with the magnetic-based stateof charge. It should also be noted that the battery may include multiplecells, each of which may be monitored with a separate magnetic sensor asdescribed above. The resultant state of charge may then be extractedthough a combination of the measurements taken by the multiple magneticsensors.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the exemplary embodiment or exemplary embodiments. Itshould be understood that various changes can be made in the functionand arrangement of elements without departing from the scope of theinvention as set forth in the appended claims and the legal equivalentsthereof.

1. A method for determining a state of charge of a battery comprising:subjecting the battery to a predetermined magnetic field such that thebattery and the predetermined magnetic field jointly create a resultantmagnetic field; sensing the resultant magnetic field; and determiningthe state of charge of the battery based on the resultant magneticfield, wherein the determining of the state of charge is further basedon an integration of current flowing through the battery with respect totime, and the determining the state of charge of the battery comprises:determining a magnetic portion of the state of charge based on theresultant magnetic field; determining a current portion of the state ofcharge based on the integration of current; and weighting the magneticportion of the state of charge and the current portion of the state ofcharge such that, as the current flow through the battery increases,influence on the state of charge by the magnetic portion is decreased.2. The method of claim 1, wherein the sensing of the resultant magneticfield comprises: subjecting a magnetically responsive component to theresultant magnetic field; and detecting an electromagnetic responsewithin the magnetically responsive component.
 3. The method of claim 2,wherein the electromagnetic response within the magnetically responsivecomponent is a current, a voltage, an electric power, or a combinationthereof.
 4. The method of claim 3, further comprising generating thepredetermined magnetic field.
 5. The method of claim 4, wherein thegenerating of the predetermined magnetic field comprises: wrapping afirst conductive wire wrapped around at least a portion of the battery;and conducting current through the first conductive wire.
 6. The methodof claim 5, wherein the magnetically responsive component is a secondconductive wire wrapped around at least a portion of the battery.
 7. Themethod of claim 4, wherein the generating of the predetermined magneticfield comprises positioning a permanent magnet within a predetermineddistance of the battery.
 8. The method of claim 7, wherein themagnetically responsive component is a Hall Effect sensor.
 9. The methodof claim 1, wherein the battery is an automotive battery.
 10. The methodof claim 1, wherein the battery is a lithium ion battery.
 11. A methodfor determining a state of charge of an automotive battery comprising:generating a predetermined magnetic field; subjecting the battery to thepredetermined magnetic field such that the battery and the predeterminedmagnetic field jointly create a resultant magnetic field; subjecting amagnetically responsive component to the resultant magnetic field;detecting an electromagnetic response within the magnetically responsivecomponent; and determining the state of charge of the battery based onthe electromagnetic response, wherein the determining of the state ofcharge is further based on an integration of current flowing through thebattery with respect to time, and the determining the state of charge ofthe battery further comprises: determining a magnetic portion of thestate of charge based on the electromagnetic response; determining acurrent portion of the state of charge based on the integration ofcurrent; and weighting the magnetic portion of the state of charge andthe current portion of the state of charge such that, as the currentflow through the battery increases, influence on the state of charge bythe magnetic portion is decreased.
 12. The method of claim 11, whereinthe battery is a lithium ion battery.
 13. The method of claim 12,wherein the determining the state of charge of the automotive battery isnot based on a voltage of the automotive battery.
 14. The method ofclaim 13, wherein the electromagnetic response within the magneticallyresponsive component is a current, a voltage, an electric power, or acombination thereof.
 15. A method for determining a state of charge ofan automotive battery comprising: generating a predetermined magneticfield; subjecting the battery to the predetermined magnetic field suchthat the battery and the predetermined magnetic field jointly create aresultant magnetic field; subjecting a Hall Effect sensor to theresultant magnetic field; detecting an electromagnetic response withinthe Hall Effect sensor; and determining the state of charge of thebattery based on the electromagnetic response.
 16. The method of claim15, wherein the automotive battery is a lithium ion battery.
 17. Themethod of claim 16, wherein the determining of the state of charge isfurther based on an integration of current flowing through the batterywith respect to time, and the determining the state of charge of thebattery further comprises: determining a magnetic portion of the stateof charge based on the electromagnetic response; determining a currentportion of the state of charge based on the integration of current; andweighting the magnetic portion of the state of charge and the currentportion of the state of charge such that as the current flow through thebattery increases, influence on the state of charge by the magneticportion is decreased.
 18. The method of claim 17, wherein the generatingof the predetermined magnetic field comprises positioning a permanentmagnet within a predetermined distance of the battery.
 19. The method ofclaim 18, wherein the electromagnetic response within the magneticallyresponsive component is a current, a voltage, an electric power, or acombination thereof.